This invention relates to electronic circuitry which cancels out the effects of drift in the amplifiers of multiplexed data monitoring systems. Methods used heretofore required the use of very low drift amplifiers in order to avoid degradation of the analog signal accuracy in multiplexed data acquisition systems. In addition the previously existing systems required periodic readjustment of the zero balance adjustment point of the amplifiers. This was particularly true in low level analog signal monitoring equipment wherein the peak to peak swing in voltage at the input to one of the multiplexer channels is only a fraction of a volt.
It has been common practice in the multiplexing art to bring the several channels of sensor information in to a sequential data sampling unit. The sampling unit monitors each input channel for a short interval, then steps on to the next channel. After all channels have been sampled, the cycle is repeated. Each sample, thus monitored, is fed to a signal amplifier for appropriate processing. In other words the bandwidth and gain of the amplifier stage has to match the parameters of both the data pickup sensors and the sampling interval of the sequential channel sampler unit. The output of the amplifier stage or stages is usually passed on to an analog-to-digital (A/D) converter stage so that the signal input information content of each channel can be formatted in digital language for analysis in a digital computer. It will be understood that the operation of the A/D converter is synchronized with the channel sensor sampling unit. In such a system the amplifiers which process the pickup sensor signals for input to the A/D converter have to remain stable. It is necessary that the amplifiers have a low drift characteristic with respect to both time and temperature. The usual approach with such a system, is to initially adjust the amplifier to a calibrated status or zero-balance condition and then periodically readjust gain and other parameters to again achieve a calibrated state.
For further information relative to the general background of the invention, reference is made to the, "Handbook of Telemetry and Remote Control", by Elliott Gruenberg, published by McGraw-Hill, New York, 1967.